It has been discovered that, to realize improved mechanical properties and processing advantages, it is desirable to reinforce synthetic rubber materials with a variety of fillers. In particular, it has been proposed to produce homogenous rubber-silica mixtures by the coprecipitation of a synthetic rubber latex and an alkali metal silicate solution, such as via the utilization of a neutral electrolyte in the presence of an acid. As U.S. Pat. No. 3,849,363 points out, these methods have the disadvantages that only mixtures with a very low silica content are obtained, to which subsequently additional fillers, for example, colloidal kaolin or activated carbon, must be added.
The present invention provides for a process of making NBR rubber silicate mixtures that have superior mechanical properties, particularly tensile strengths, without the addition of other fillers.